1. Field of the Invention
The present invention pertains to optical testing devices and in particular to optical testing devices for measuring the optical coupling coefficients of materials. In even greater particularity, the present invention pertains to the measurement of optical coupling coefficients through the thermal expansion of material while under a stress load that is monitored.
2. Description of the Prior Art
When optical radiation falls on a substance, energy is absorbed. The ratio of absorbed to incident flux is the optical absorptance. Absorptance depends upon the properties of the optical radiation, such as wavelength, intensity, duration, etc. as well as those of the absorber, such as reflectance, etc. When the intensity is sufficient to alter absorptance, the term coupling coefficient is used to express the fractional energy absorbed in a given time interval. The absorbed energy is converted to heat within the absorbing sample. Coupling coefficients are frequently measured by observing the resultant temperature rise, often by direct instrumentation of the absorber. In general, the prior methods of measuring coupling coefficients are referred to as calorimetric techniques.
Accurate measurement of optical coupling coefficients becomes urgent as the use of high-power laser systems becomes widespread. Ever more frequently, new optical and structural materials undergo intense irradiation at a variety of wavelengths. Problems due to laser-induced fatigue or failure can be avoided or minimized in system design if material response to optical radiation is known.
The hereafter described method and apparatus is a fundamentally different concept for measuring coupling coefficients. The basis is the thermo-mechanical expansion or contraction of the absorber. This provides a direct measure of the total energy absorbed, without considering the temperature distribution in the absorber itself.